Measuring device and methods for characterization of a radiation field

ABSTRACT

A radiation field measuring device for the characterization of a radiation field is disclosed. The measuring device may include a detector device and a reconstruction device. The detector device may have at least one detector camera, which contains at least one detector array arranged for the image recording of scattered radiation in a multiplicity of lateral directions that deviate from the longitudinal direction. The reconstruction device may be configured for the tomographic reconstruction of a field density of the scattered radiation in the radiation field.

CROSS REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

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STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

The invention relates to a radiation field measuring device and methodsfor characterizing a radiation field of electromagnetic radiation, moreparticularly of laser radiation.

It is generally known that the effectiveness and exactitude ofradiation-based methods, e.g., for material processing or measurementpurposes, depend upon the geometric properties and/or field propertiesof the radiation field that is used in the radiation-based method. Forexample, the effectiveness of material processing with laser radiationis influenced by the formation of a focus of the laser radiation on thesurface of the material. Hence an examination (measurement) of radiationfields for ascertaining the properties thereof and optionally forguiding a radiation source such that the radiation field is generatedwith prespecified properties is generally of interest.

Standard methods for examining radiation fields comprise invasivemethods and non-invasive methods. Invasive methods, such as capturingthe radiation field directly with a camera, have the disadvantage thatthe application thereof alters the radiation field to be examined. As aresult, a desired effect of the radiation field may be adverselyimpacted or even temporarily suspended. In online monitoring of lasercutting or welding units, for example, impacting or damaging the lightdistribution of a working beam should be avoided. Even if only part ofthe radiation field to be examined is separated and examined separatelyfrom a main beam (see DE 101 49 823 A1, for example), the optics usedfor the separation may adversely impact the application of the main beambecause of dirt, for example.

Furthermore, invasive methods are limited to examining low power densityradiation fields. Optics, for example mirrors, prisms, filters and/orlenses, in the beam path of the radiation field to be examined may bedestroyed in the case of high power densities. For this reason, it isgenerally impossible to examine, for example, the radiation fielddirectly in the focus of laser radiation with an invasive method.Lastly, invasive methods for measuring a radiation field, particularlyin the case of monochromatic radiation (in the case of a CW laser, forexample) tend to cause artefacts due to diffraction at defects orcontaminations on the optics, e.g., on lens surfaces. This can lead tointerferences, is nearly impossible to avoid, and adversely impacts theexactitude of the measurement.

Non-invasive methods have the advantage that they can be usedparticularly with high radiation intensities and that the radiationfield to be examined is not affected by the measurement. U.S. Pat. No.8,988,673 B2, for example, describes a non-invasive method in which thescattered light of a laser beam is recorded with a camera during passagethrough a gas in order to measure the shape of the pharoid beam (of theentire beam bundle). 2D scattered radiation images, which representprojections of the intensity distribution of the laser beam on planesparallel to the beam direction, are measured with this method.

The method according to U.S. Pat. No. 8,988,673 B2 has the disadvantagethat neither transaxial 2D sections nor 3D volume reconstructions of thelaser beam can be ascertained. A sequence of 2D projections of theradiation field, for instance by the repeated movement of a camera alonga prespecified linear profile, could be achieved with the methodaccording to U.S. Pat. No. 8,988,673 B2. However, measuring a singlelight pulse would not be possible with this method.

A general problem of standard techniques for examining radiation fieldslies in the fact that these techniques are limited to the ascertainingof individual properties and are not suitable for a completecharacterization of the radiation field by just one measurement. Moreparticularly, there are no known non-invasive methods for simultaneouslyascertaining a plurality of parameters (e.g., intensity distribution,caustics, M²-parameters, beam propagation, wave front, wavelength andpolarization properties, and/or beam shape) of the radiation field.

The simultaneous ascertainment of several properties of a radiationfield has thus far only been possible through the combination orsequential application of different measurement methods, which increasesthe complexity of the examination. Furthermore, several measurements canexert negative influences on each other by their respective impacts onthe radiation field, thereby adversely affecting or even precluding anexact representation of the radiation field. Lastly, the application ofsequential measurements on invariable radiation fields would be limitedand unsuitable for the examination of, for example, individual laserpulses or transient light distributions.

SUMMARY OF THE INVENTION

The object of the invention is to provide an improved radiation fieldmeasuring device and an improved method for characterizing a radiationfield of electromagnetic radiation, particularly laser radiation, withwhich disadvantages of standard techniques are avoided. The inventionshould more particularly make it possible to ascertain more propertiesof a radiation field and/or to characterize the radiation field in anon-invasive manner with greater local resolution, precision and/orreproducibility, and/or to give rise to new applications of thecharacterization of a radiation field. In particular, an as complete aspossible measurement and reconstruction of the radiation field shouldfurthermore be achievable, ideally by using only one measurement method.More particularly, this should be achievable with an individualmeasurement or with several time-resolved individual measurements. Theradiation field measuring device should furthermore be distinguished bya simplified technological design and/or an enhanced range ofapplications.

The method disclosed here is based on the capturing of scattered (stray)radiation that a radiation field generates in a medium. Applications ofthe invention lie in the monitoring and/or controlling of radiationsources, more particularly laser sources, for material processing, andof radiation-based methods, e.g., for material processing or measurementpurposes.

Accordingly, a radiation field measuring device is described, which isconfigured for characterizing a radiation field that passes through amedium in a longitudinal direction. The radiation field measuring devicemay include a detector device having at least one detector camera, whichcontains at least one detector array arranged for the image recording ofscattered radiation that is generated in the medium by the radiationfield and is directed in a multiplicity of lateral directions thatdeviate from the longitudinal direction. The radiation field measuringdevice may also include a reconstruction device, which is configured forcharacterizing the radiation field on the basis of image signals of thedetector device. In this radiation field measuring device, thereconstruction device may be configured for the tomographicreconstruction of a field density of the scattered radiation (3) in theradiation field.

A method for characterizing a radiation field is also disclosed. Theradiation field may pass through a medium in a longitudinal direction,using a radiation field measuring device. The method may include imagerecording, by means of the detector device, of scattered radiation,which is generated in the medium by the radiation field and is directedin a multiplicity of lateral directions that deviate from thelongitudinal direction. The method may further include characterizingthe radiation field with the reconstruction device using image signalsof the detector device, Within this method, the reconstruction devicemay carry out a tomographic reconstruction of a field density of thescattered radiation in the radiation field.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the invention shall be described inthe following, with reference to the appended drawings. Shownschematically are:

FIG. 1: a preferred embodiment of the radiation field measurement deviceaccording to the invention;

FIG. 2: an arrangement of reflector sections of the radiation fieldmeasurement device according to FIG. 1;

FIG. 3: an arrangement of detector cameras for recording scatteredradiation images;

FIGS. 4 to 14: features of further embodiments of the radiation fieldmeasuring device according to the invention having different variants ofa deflector device;

FIG. 15: features of a radiation field measuring device having a beamrotator; and

FIG. 16: a flow diagram with an illustration of features of preferredembodiments of the method according to the invention.

DETAILED DESCRIPTION

According to a first general aspect of the invention, said object isachieved with a radiation field measuring device (also called ascattered radiation tomograph) for the characterization of a radiationfield that passes through a medium in a longitudinal direction (beamdirection), which device comprises a detector device and a (tomography)reconstruction device.

The detector device has at least one detector camera with at least onedetector array, which is arranged for the image recording of scatteredradiation, which is generated in the medium by the radiation field anddirected in a multiplicity of lateral directions (rotary directions)that deviate from the longitudinal direction.

According to the invention, the reconstruction device is configured forthe characterization of the radiation field by means of acomputer-tomographic, locally resolved reconstruction (denoted astomographic reconstruction here) of a field density (energy or powerdensity, spatial distribution) of the scattered radiation in theradiation field by using image signals of the detector device. Thecharacterization of the radiation field generally comprises thedetermination of the field density of the scattered radiation andpreferably the determination of beam parameters, in particular geometricbeam parameters and/or field beam parameters, of the radiation fieldand/or the determination of a distribution of scattering particles inthe medium.

The field density of the scattered radiation is a function of theintensity distribution in the radiation field and thus more particularlyenables the provision of the sought-after beam parameters. In the eventthat Rayleigh scattering is generated from a monochromatic radiationfield, the scattered radiation is proportional to the intensitydistribution in the radiation field. For the case of a polychromaticradiation field, a proportionality also arises under the condition thatthe detected spectral distribution does not vary spatially in themeasurement volume of the radiation field and more particularly that theintensity I of the radiation field can be factored according to I(λ,r)=I₁(λ)*I₂(r). This condition can be fulfilled, for example, byequipping the detector device with a spectrally selective filter, whichlets a partial spectral range of the radiation field through.

If the intensity of the scattering of the radiation field is a linearfunction of the intensity of the radiation field, the tomographicreconstruction then gives the 2D or 3D intensity distribution of theradiation field on the basis of the scattered radiation, excluding acalibration factor. In the case of other scattering processes, aquantitative correlation between the field density of the scatteredradiation and the intensity distribution in the radiation field can alsobe established by calibration measurements or by applying scattermodels.

According to a second general aspect of the invention, said object isachieved by the use of the radiation field measuring device according tothe first general aspect of the invention for controlling a focus of theradiation field, capturing a temporal drift of an intensity profile ofthe radiation field, characterizing the radiation field of high energylasers, laser-supported material processing in cutting and joiningtechnologies, manufacturing in semiconductor technology, or therapyand/or surgery using laser radiation, and/or monitoring and/orcontrolling radiation-based processes, e.g., in controlling a radiationsource, more particularly a laser source. According to the second aspectof the invention, in particular a control device for a radiation source,more particularly one comprising the tomography/reconstruction device,is deemed independent subject matter of the invention.

According to a third general aspect of the invention, said object isachieved by a method for the characterization of a radiation fieldpassing through a medium in a longitudinal direction using a radiationfield measuring device according to the first general aspect of theinvention, wherein provision is made for an image recording of scatteredradiation, which is generated in the medium by the radiation field anddirected in a multiplicity of lateral directions that deviate from thelongitudinal direction, by means of the detector device, and for acharacterization of the radiation field with the reconstruction deviceusing image signals of the detector device, and wherein thereconstruction device carries out a tomographic, locally resolvedreconstruction of the field density of the scattered radiation in theradiation field.

The invention in general enables the characterization of a directedradiation field of incoherent radiation or coherent radiation (laserradiation). Provision is preferably made for the characterization oflaser radiation, since this facilitates a reconstruction of the fielddensity of scattered radiation with a high signal-to-noise ratio. Theradiation field can be a continuous radiation field (continuous mode, CWmode) or a pulsed radiation field (pulse mode), wherein the powerdensity of the scattered radiation is reconstructed as a field densityin the continuous mode and the energy density of the scattered radiationis reconstructed as a field density in the pulse mode.

The scattered radiation is generated by the radiation field in themedium, which generally comprises a diffusing substance, particularly atleast a gas (or vapor), for example air or other gaseous process mediumor scattering gas, a liquid, a solid, a plasma, or a particle-containingcomposition such as a colloidal solution, an aerosol, smoke, or anemulsion. Depending upon the nature of the diffusing medium, thescattered radiation will be generated by, for example, Rayleighscattering, Tyndall scattering, Mie scattering or scattering on freecharge carriers. These scattering mechanisms are each distinguished by aspecific distribution of the scattered radiation (e.g., shape of thescattering lobe or orientation relative to the longitudinal direction ofthe radiation field), which can be taken into account during thetomographic reconstruction of the field density. A calibrationmeasurement can be used to determine the spatial characteristics of thescattered radiation.

The image signals of the detector device (scattered radiation images)provide projections of the scattered radiation in the captured lateraldirections on the at least one detector array. The reconstruction deviceis configured to obtain, by means of tomographic reconstruction, atleast one sectional image of the scattered radiation in the radiationfield from the scattered radiation images, which are recorded fromseveral different directions (the captured lateral directions)corresponding to a number of projections. The sectional image of thescattered radiation represents the field density of the scatteredradiation in the radiation field, more particularly the spatialdistribution of the scattered radiation in the radiation field, which isa qualitative and quantitative measure for the field distribution of theradiation field. The reconstruction device provides a three-dimensionalmodel of the field distribution of the radiation field.

The limitations of standard techniques are advantageously avoided by theuse of the scattered radiation tomograph in that the scattered radiationarising in the medium anyway based on, e.g., Rayleigh scattering of theradiation field or fluorescence on atoms or molecules of the medium, isused in order to characterize the radiation field in a comprehensivemanner and with just one measurement. In particular, the imperfectionsof the standard scattered light 2D imaging method according to U.S. Pat.No. 8,988,673 B2 are remedied by the tomographic reconstruction of theradiation field, and a complete reconstruction of the radiation field isachieved in the measurement section of interest without disturbing theradiation distribution.

The following advantages of the invention also arise. A scatteredradiation tomograph operates in a contact-free (i.e., non-invasive)manner so that the radiation distribution to be examined is notinfluenced by the measurement. The fact that dust particles and defectson optical components cannot interfere is particularly advantageous,since there are no such optical components in the beam path of theradiation field to be examined. Particularly high radiation intensitiescan be measured without damaging components of the scattered radiationtomograph. As an alternative, the scattered radiation tomograph can beconfigured for an invasive mode, for example if the examined radiationfield is to be rotated in the optical setup used for the image recordingof the scattered radiation or if the examined radiation field is to besplit off from a main beam.

The invention enables a comprehensive measurement of the radiationfield, in particular a three-dimensional reconstruction of the intensityprofile of a radiation field in a measurement volume, and the derivationof a multiplicity of beam parameters from the same. The measurement cantake place in a manner free of artefacts, more particularly free ofinterferences, silhouettes, and/or diffractions. The scattered radiationtomograph enables the reconstruction of the intensity profile of theexamined radiation field, even for transient radiation fields and inparticular for one-time radiation pulses. Advantageously, a plurality oftransient phenomena of the radiation field can also be capturedsimultaneously in a measurement volume. The scattered radiationtomograph has a considerably simplified construction compared to thecombination of standard measurement setups that would be required for acomprehensive characterization of the radiation field.

In contrast to the composite image measured in U.S. Pat. No. 8,988,673B2, the invention advantageously provides a tomographic reconstructionof the radiation fields or the beam parameters thereof. Thecharacterization of the radiation field is independent of theobservation direction, since provision is made anyway for capturing thescattered radiation images from multiple angles for the tomographicreconstruction. The complete three-dimensional reconstruction of theintensity profile in a measurement section makes it possible to derivefreely selectable two-dimensional intensity profiles along any sectionalplane through the medium in the measurement section.

A further advantage of the invention lies in the fact that thecharacterization of the radiation field is made possible for radiationin different wavelength ranges. In this case the term “radiation” refersin particular to electromagnetic radiation with a wavelength in thex-ray, UV, VIS, NIR, IR or microwave range. The detector device ispreferably configured for an image recording of the scattered radiationwith a wavelength in the x-ray, UV, VIS, NIR, IR or microwave range ineach respective case. Particular preference is given to characterizinglaser radiation with a wavelength in the UV, VIS, NIR, or IR range.However, specific advantages also arise for other wavelength ranges. Onthe basis of the emission of the radiation arising in the recombination,it is thus possible to measure intensity distributions of ionizingradiation, for instance x-ray or XUV/UV radiation, which would damage ordestroy standard radiation detectors.

Soft x-ray radiation, which is characterized with the method accordingto the invention, preferably has an energy of 0.1 to 1 keV. For example,for a wavelength in the range of 1 nm to 10 nm, which correspondsroughly to 1000 eV to 100 eV, the scattering and absorption in air isalready comparable to the scattering of visible light in air. Thecharacterization of x-ray radiation is of interest in, for example,applications in the semiconductor industry, in particular formicrolithography, for which there are not any suitable non-invasive beamdiagnosis methods as yet.

The reconstruction device is preferably configured for non-analytical,in particular algebraic or statistical, tomographic reconstruction ofthe field density of the scattered radiation. Particular preference isgiven to the tomographic reconstruction of the field density of thescattered radiation comprising an iterative algorithm.

Tomographic reconstruction with a non-analytical method has advantagesover analytical methods in terms of the achievable quality andquantifiability of the reconstruction result, in particular because theyare calculated in a more artefact-free manner and with better spatialresolution. For example, non-analytical methods can considerably reducethe sampling artefacts in the reconstruction result that would otherwisebe expected for the sought-after low projection number. Furthermore,they generally enable all of the physical effects arising during theimage acquisition and typically degrading the image quality to be takeninto account. For example, these can be the characteristics of theimaging system (the so-called point spread function, PSF) or perhaps theoccurrence of reflection/scattered radiation. Common to allnon-analytical methods is the fact that they discretize the space, andconsequently also the reconstruction result and the measurement data, atthe outset. This means that the reconstruction result to be determinedis broken down into a multiplicity of three-dimensional voxels by thediscretization of the space, and that the measurement data areaccordingly broken down into a multiplicity of two-dimensional pixels.

Algebraic reconstruction methods constitute a first subgroup ofnon-analytical reconstruction methods. They invert a linear equationsystem or determine the pseudoinverse (Moore-Penrose inverse) thereof.Because of the high dimensionality of the assigned task, the numerousalgebraic reconstruction methods are implemented iteratively, forexample with ART, MART, or SMART algorithms.

Tomographic reconstruction is preferably performed with a secondsubgroup of non-analytical reconstruction methods, namely thestatistical reconstruction methods. These methods are likewiseessentially implemented iteratively. In particular, they have advantagesin terms of reconstruction based on noisy image recordings of scatteredradiation (measurement data (y)). For example, the Poisson noise of themeasurement data, which can arise as a result of the low intensity ofthe scattered radiation, can be taken into account implicitly.Statistical methods are based on the formulation of a high-dimensionalgoal or cost functional F(f), which assumes a minimum, ideally a globalone, for a specific choice of voxel values. The totality of these samevoxel values for which the cost functional is minimized represents thereconstruction result f. There are numerous applicable statistical,iterative reconstruction methods according to the type of formulation ofthe goal functional and the type of iterative calculation rule(algorithm) for the search of the goal functional minimum.

The goal functional F(f) consists of at least two components. In thecontext of the noise characteristics of the measurement datay, thetomographic data mismatch term L(y, A f) that formulates the imagingrequires that the forward projections of f, calculated by applying asystem matrix A to f, A f, coincide with measurement values y. Thesystem matrix A thus formulates the measurement geometry and basicallytakes all physical effects of the measurement value creation intoaccount. The maximum likelihood term, which takes the typically-arisingPoisson noise characteristics of the scattered radiation into account,is preferably used as the tomographic data mismatch term.

The second component of the goal functional is provided because theformulation of the goal functional solely by the data mismatch term is aso-called ill-posed problem, which generally leads to a noiseamplification of the reconstruction result in course of the iterativeminimization process. The goal functional is therefore preferablysupplemented with a Bayesian regularization term R(f), which is based onprior knowledge of spatial relationships of the voxel values of thereconstruction result.

Statistical tomographic reconstruction using scattered radiation imagesis preferably carried out in a manner analogous to the tomographicreconstruction of emission tomographic measurement data, which isdescribed in U.S. Pat. No. 8,559,690. Accordingly, the goal functionalto be minimized is preferably supplemented by a third term. The latteris an Lp-norm term with (0≤p<2), in particular an L1-norm term: ∥T ^(T)f∥₁, which amplifies the sparseness, or else at least thecompressibility, off. Because the compressibility off is generally notprovided in the spatial domain, f is transformed by means of T^(T) intoa sparse or else at least compressed representation, for instance byapplying a three-dimensional wavelet transformation, which isfurthermore selected such that it is as incoherent as possible withrespect to the system matrix A. The compressive sensing paradigm, whichenables a substantial reduction of the number of individual measurementsof the scattered radiation at different angles needed for anartefact-free reconstruction, is advantageously associated with theplugging-in of this term.

The goal functional to be minimized is therefore preferably formulatedas follows:

F( f )==L( y, A f )+α∥ T ^(T) f∥ ₁ +βR( f ),

wherein α and β are factors that specify the effect of the respectivegoal functional components. The algorithm to be used for minimizing thegoal functional can be freely chosen, provided that it adequatelyaccounts for the numerically demanding L1-norm term. This applies inparticular to the requirement that the voxel values must fulfill theboundary condition f≥0. Preference is therefore given to using aso-called “Alternating Direction Method of Multipliers” (ADMM)algorithm.

The characterization of the radiation field preferably comprises thedetermination of the beam parameters in a particle-free medium. However,under practical application conditions dust particles in the measurementsection can cause artefacts and interferences of the reconstruction. Forcapturing Rayleigh scattering, provision is therefore preferably madefor eliminating dust and Mie scattering events in the medium. Accordingto a particularly advantageous embodiment of the invention, a purelystatistical approach is preferably used for detecting and if need beeliminating dust and Mie scattering events in the medium. With thisapproach, several scattered radiation images are recorded sequentiallyand artefact-bearing scattering events arising from particles in themedium are eliminated by a statistical analysis of the series ofscattered radiation images. This approach is thus based on effectivereconciliation of several sequential individual measurements, and itadvantageously does not need any predefined parameters. According to analternative variant of the invention, provision can be made forreconstruction of the field density of the scattered radiation, withparticles in the medium taken into account that would otherwise lead toartefacts of the reconstructed field density.

For a radiation field that remains stationary for a sufficient time, themeasurement associated with each projection direction could be performedmultiple times. If the repetition frequency of the multiple measurementis adapted to the mean movement velocity of the dust particles,transient scattering events will be effectively eliminated by a medianoperation carried out in a pixelwise manner. The projection imagessupplied to the tomographic reconstruction will simultaneously bedenoised.

The application of the invention is not limited to the use ofnon-analytical methods. Alternatively, use can be made of analyticalmethods, which are characterized by the fact that they perceivereconstruction results and measurement data as continuous functions anddirectly solve an integral equation that implicitly simplifies theprojection process. Examples of such include filtered back projection(FBP) and convolution back projection (CBP).

The tomographic reconstruction can advantageously be carried out suchthat the illumination background determined using a referencemeasurement is implicitly taken into account in the scope of a forwardand backward projection process of the tomographic reconstruction ratherthan subtracted from the scattered radiation images (projections). Thecurrent illumination background can comprise, for example, an exteriorillumination if the medium in the measurement volume cannot becompletely screened from outside incident light, and/or secondaryscattering of the Rayleigh scattering on objects near the measurementsetup. In the latter case, the secondary scattering, which is overshoneby the radiation field itself in the projection area of the radiationfield, is estimated in this projection area by interpolation.

The lateral directions in which the scattered radiation images arerecorded run perpendicular to the longitudinal direction, wherein inthis case they represent the radial directions, or at an angle greaterthan or less than 90° relative to the longitudinal direction.

According to another preferred embodiment of the invention, the at leastone detector array for the image recording of scattered radiation isarranged such that the lateral angles are distributed in such a way thatthe components of the recorded scattered radiation span a measurementrange of 180° to 360° perpendicular to the longitudinal direction.

If the scattering medium weakens the scattered radiation en route to theradiation field measuring device homogeneously or inhomogeneously insideand/or outside of the radiation field, the scattered radiation ispreferably captured from lateral directions selected such that therespective components thereof are distributed over 360° perpendicular tothe longitudinal direction of the radiation field. If the weakening ofthe scattered radiation by the scattering medium en route to theradiation field measuring device is negligible, the scattered radiationis preferably captured from lateral directions selected such that therespective components thereof are distributed over 180° perpendicular tothe longitudinal direction of the radiation field.

Preference is given to measuring the scattered radiation in lateraldirections, the components of which are uniformly distributed over themeasurement range perpendicular to the longitudinal direction, except inthe case of an even number of lateral directions, the components ofwhich are to be distributed over 360° perpendicular to the longitudinaldirection. In this case, they are preferably distributed unevenly overthe measurement region. In this manner the recording of redundant imageinformation of the scattered radiation is advantageously avoided, andthe number of lateral directions that are required for a specificapplication of the invention for characterizing the radiation field canbe minimized.

Advantageously, the radiation field can be characterized using scatteredradiation images that were recorded in only two different lateraldirections (lateral angle not equal to 180°, preferably equal to 90°).As an alternative, scattered radiation images are recorded along atleast three lateral directions, more particularly at least four (for ameasurement range of) 180° or at least five (for a measurement range of360°) lateral directions and undergo tomographic reconstruction.

According to a preferred variant of the invention, the detector devicefor the image recording of scattered radiation is arranged perpendicularto the longitudinal direction. In this case, advantages can arise due tothe available space and the alignment of the detector device relative tothe longitudinal direction. According to an alternative variant of theinvention, the image recording can take place at an angle greater thanor less than 90° relative to the longitudinal direction, whereinadvantages arise due to an increase of the scattering intensity if theangle of the lateral direction of the image recording relative to thelongitudinal direction decreases or increases.

If the detector device is configured according to another embodiment ofthe invention for a spectrally selective image recording of thescattered radiation, i.e., scattered radiation images are only recordedwithin a limited spectral range with the detector device, advantages canarise in terms of an improved suppression of interfering externalradiation and an improved signal-to-noise ratio of the reconstruction.The detector device can be equipped with, for example, at least onesuitable filter, which permits the passage of the desired spectralrange. Another advantage of the spectrally selective image recording ofscattered radiation lies in the simplification of the reconstruction ofpolychromatic radiation fields.

Advantageously, there are various possibilities for reconstructing thefield density in a measurement section of the radiation field to beexamined. According to a first variant, a planar layer-like section(layer section) of the radiation field is captured. Because the layerhas a finite thickness, the reconstructed field density is captured as avolumetric quantity. The layer section can be perpendicular or inclinedrelative to the longitudinal direction. The thickness of the layersection is preferably selected such that the field density within thelayer is approximately constant. In this variant, the reconstructiondevice is configured for the tomographic reconstruction of a transverseor inclined layer of the field density of the scattered radiation in thelayer section with finite thickness of the radiation field.

A conventional, invasive beam profile measuring device generallycaptures a two-dimensional intensity distribution of the radiation fieldperpendicular to the longitudinal direction thereof. With the layersuitably oriented, the inventively reconstructed volumetric fielddensity of the scattered radiation can also be transformed into atwo-dimensional intensity distribution by integrating the field densityof each voxel in the longitudinal direction of the radiation field andthen multiplying by a conversion factor.

Determining the two-dimensional intensity distribution from just asingle reconstructed layer of the field density of the scatteredradiation has advantages in terms of less apparatus and calculationcomplexity compared to a measurement and reconstruction extending over aplurality of layers. Accordingly, the detector device can preferablycomprise linear detector arrays with which linear scattered radiationimages are recorded. This advantageously gives rise to a simplifiedconstruction of the detector device.

According to a second variant, a three-dimensional, typicallycylinder-shaped or truncated cone-shaped volume section of the radiationfield is captured, which consists of a plurality of layers of finitethickness or of voxels arranged in another volumetrically suitable way.In this case, the reconstruction device is configured for thetomographic reconstruction of the field density of the scatteredradiation in the entire three-dimensional volume section, which formsitself in each dimension by the juxtaposition of voxels. In thisembodiment of the invention, the detector device preferably comprisesplanar detector arrays. The volume section is more particularly composedof at least two juxtaposed layer sections, preferably juxtaposed in alongitudinal direction. The volume section can be characterized by afield density that is variable in a longitudinal direction.

Advantageously, there are also different possibilities for configuringthe detector device. According to a first variant, the detector devicecan comprise a plurality of detector cameras, which are each equippedwith at least one detector array. In this case, an associated detectorcamera is provided for each lateral direction in which a scatteredradiation image is to be recorded. Each detector camera produces ascattered radiation image for one of the lateral directions so thatadvantages arise if the scattered radiation images are to be recordeddirectly and without additional optical elements.

According to a second variant, the detector device can comprise a singledetector camera, which contains a multiplicity of detector arrays, whichare each arranged in one of the lateral directions for the imagerecording of scattered radiation. The detector arrays can comprise, forexample, separate arrays, e.g., CCD chips, or preferably sections of acommon array, e.g., CCD chips. This embodiment of the invention has theadvantage of a simplified construction and operation of the detectordevice.

If according to another preferred embodiment of the invention provisionis made of a deflector device, which is arranged for deflecting thescattered radiation along the multiplicity of lateral directions ontothe plurality of detector cameras or onto the single detector camera,advantages can arise in terms of the positioning of the at least onedetector camera, in particular at a distance from and/or jointly on aside of the radiation field. The deflector device comprises opticalelements, particularly preferably at least one catoptric element(particularly mirrors) and/or at least one dioptric element(particularly prisms and/or lenses), with which the beam path of thescattered radiation from one of the lateral directions to the associateddetector camera is spanned in each case. The optical elements can bedesigned for displaying the scattered radiation on the at least onedetector camera.

The deflector device can advantageously comprise a plurality ofcatoptric elements, in particular a plurality of reflector sections,which are each arranged for deflecting the scattered radiation along oneof the lateral directions toward one of the detector arrays. Thereflector sections are preferably individual, planar or imaging mirrorsor are connected to an axicon reflector, which is arrangedaxial-symmetrically to the longitudinal direction. The individualmirrors have advantages in terms of the optimizable alignment of theindividual beam paths, whereas the measurement setup is advantageouslysimplified with the axicon reflector.

If the detector device comprises a single detector camera with aplurality of detector arrays, a collection reflector is preferablyprovided as a further catoptric element, which collects beam paths fromthe lateral directions via the reflector sections and directs them tothe detector camera. The collection reflector has the advantage ofsimplifying the alignment of the detector camera relative to thereflector sections.

According to another preferred embodiment of the invention, theradiation field measuring device can be equipped with a beam rotator,which has a rotatable prism, in particular a Dove prism, and/or arotatable mirror and which is configured for rotating the radiationfield about the longitudinal direction. In this case, the detectordevice contains a single detector camera, which is arranged for theimage recording of scattered radiation. For the recording of scatteredradiation images in the multiplicity of lateral directions, theradiation field is rotated with the beam rotator into various rotationpositions relative to the detector camera. It should be noted that thisembodiment is designed for a non-destructive measurement, i.e., itpermits the simultaneity of measurement on the radiation field andprimary application of the radiation field. However, this embodiment isonly usable with radiation field intensities that permit the use of therotatable prism and/or mirror.

According to a particularly preferred embodiment of the invention, thecharacterization of the radiation field comprises the determination ofbeam parameters directly from the tomographically reconstructed fielddensity of the scattered radiation. Preference is given to providing ananalyzer device, which is part of the reconstruction device or arrangedseparately therefrom and which determines at least one beam parameter ofthe radiation field from the field density of the scattered radiation.Advantageously, the analyzer device can be used to calculate at leastone of the following beam parameters: field-beam parameters, e.g., thepulse energy or pulse energy density of the radiation field in the caseof pulsed radiation, the field density of the radiation field in thecase of continuous radiation, coherence properties of the radiationfield, wave fronts of the radiation field, Rayleigh lengths of theradiation field, or diffraction indexes, M² parameters and beampropagation factors k of the radiation field, and/or geometric beamparameters such as geometric properties of the radiation field, inparticular beam diameter, divergence angle and/or beam shape, propertiesof the beam waist of the radiation field, in particular the radius,position along the longitudinal direction, and/or shape of the focus intransaxial section, and/or the spatial location of the radiation fieldin the medium. The beam parameters can advantageously be determinedindividually, in subgroups, or in their entirety from a singlemeasurement of the radiation field.

If the analyzer device is configured according to another embodiment ofthe invention for a continuous determination of the at least one beamparameter and the temporal stability thereof, advantages arise in termsof the continuous monitoring of the radiation field and optionally thecontrolling of a beam source for generating the radiation field.

According to another variant of the invention, the analyzer device canbe configured for calculating beam properties, which are derived fromthe determined beam parameters. A preferred example is the calculationof beam propagation, in particular by means of a wave front analysis. Inan examination of the radiation field in a measurement section spacedapart from a radiation field action site on a material, calculating thebeam propagation enables beam parameters to be determined at the actionsite. For example, the focus of the radiation field can be characterizedand the position of the focus can be ascertained, even if the scatteredradiation images used for the tomographic reconstruction are recordedoutside of the focus.

According to another advantageous embodiment of the invention, theradiation field measuring device can be equipped with a particle removaldevice, which is configured for providing the medium in a particle-freestate in the measurement section of the radiation field measuringdevice. The particle removal device has the advantage of removing dustparticles from the measurement section, which could otherwise causeartefacts and interferences of the reconstruction. Advantageously,various technical measures for removing dust particles are available,for example electrostatic filters, mechanical filters for generating aconstant particle-free medium flow through the measurement section,and/or purging gas sources for the provision of purified media or of apurging gas for the measurement section.

According to another application of the invention, the reconstruction ofthe field density of the scattered radiation can be used to determine avolumetric particle distribution in the radiation field. In this case,interim results of the inventively applied reconstruction advantageouslyprovide information on the presence, the spatial distribution, the shapeand the size distribution of scattering particles in the examinedradiation field. Particle spectra can in turn be derived from thelatter.

According to a particularly preferred embodiment of the method accordingto the invention, provision is made for a monitoring and/or controllingof a radiation source with which the examined radiation field isgenerated. Determined beam parameters of the radiation field are used tomonitor the operation mode of the radiation source and optionally toadjust it and/or stabilize it by means of a control circuit. Theradiation source is preferably a laser source, which is configured, forexample, for a laser-supported material processing in cutting andjoining technologies or for a manufacturing technique in semiconductortechnology or a laser-supported surgical technique.

The setting and optional regulation of the radiation source cancomprise, for example, actuation of a focusing device of the radiationsource according to the determined position of the focus of theradiation field along the longitudinal direction in such a way that thefocus is set to a predefined working position, e.g., on the surface of amaterial to be processed.

As an alternative or in addition, the radiation source can contain asetting device with which beam parameters of the radiation field arealterable, wherein in this case the setting device is controlledaccording to a determined beam parameter, particularly preferablyaccording to an intensity profile of the radiation field along thelongitudinal direction, particularly in the focus of the radiationfield.

Features of preferred embodiments of the invention shall now bedescribed in the following, with reference to the examination ofradiation fields of light (light fields) as an example, which comprise,for example, continuous or pulsed laser light or non-coherent light, inparticular in the UV, VIS, or IR wavelength range, with tomographicreconstruction using scattered light images. Accordingly, opticalelements of the radiation field measuring device comprise in particularmirrors, lenses and/or prisms. However, the practical implementation ofthe invention is not limited to the characterization of light, but isalso possible for radiation fields of other wavelengths, for examplex-ray radiation. In these cases, the optical elements are replaced asneeded with suitable elements for beam deflection and/or imaging such asx-ray optics, multilayer mirrors or mirror arrangements with grazingincidence, for example, and the detector camera(s) comprise(s), forexample, CCD cameras with convertor layers or cameras with imageconvertor tubes.

The radiation field measuring device and methods for operating the sameare also described, particularly in terms of the collection of scatteredlight images and the construction of the detector device. Details of thereconstruction method are achievable in the manner known from standardmethods of emission tomography, particularly according to U.S. Pat. No.8,559,690.

In preferred embodiments of the invention, scattered light images can becollected using a deflector device having catoptric and/or dioptricelements. Accordingly, features of embodiments of the invention havingcatoptric elements are also achievable using dioptric elements (and viceversa). For example, the effects of reflector sections are achievableusing optical lenses. However, catoptric elements such as mirrors, forexample, have advantages because they do not have any color errors andcan be more easily adapted to an elliptical arrangement with beamdiversion. Compared to lens or prism assemblies or to the use of asingle detector camera, multi-mirror arrangements having severalreflector sections furthermore have the advantage of a high spatialangle coverage.

FIG. 1 shows, in schematic form, a first embodiment of the radiationfield measuring device 100 according to the invention for thecharacterization of a light field 1, having a detector device 10, adeflector device 30 and a reconstruction device 20. In this embodiment,the detector device 10 comprises a single detector camera 11. Theradiation field measuring device 100 is provided for thecharacterization of the light field 1 of a laser beam, which isgenerated for the purpose of material processing with a laser source210, e.g., a CO₂ laser, a Nd-YAG laser or a disc laser, and focused onthe surface of a workpiece 220. For example, the mode structure andoutput of the light field 1 and the position of a focus on the surfaceof the workpiece 220 should be ascertained and optionally controlled.The light path of the light field 1 runs with a beam direction(designated here as longitudinal direction z) through a measurementsection 4 containing a medium 2 such as air, for example. In themeasurement section 4 the light field 1 has a cross sectional dimensionof, for example, 10 μm to 10 cm, typically 1 mm to 10 mm.

FIG. 1 shows the examination of the light field 1, which is generateddirectly by the laser source 210. As an alternative, if the fielddensity of the radiation is sufficiently low, a beam splitter can beused to split the light field 1 off from a main beam, which is directedto the material to be processed.

The light field 1 is scattered on the molecules of the medium 2, thusgenerating scattered light 3. The scattered light 3 is radiated in andopposite and laterally to the longitudinal direction z, with componentsin the x-y plane. In the case of predetermined lateral directions, partof the scattered light 3 is collected by the deflector device 30 (seeFIG. 2A) and directed toward the detector camera 11 of the detectordevice 10. With the detector camera 11, scattered light images 6 of thescattered light 3 generated in the light field 1 are recorded along thelateral directions (see FIG. 2B).

The deflector device 30 comprises reflector sections 31 and a collectionreflector 32 in the form of planar mirrors, which in the exampleillustrated are inclined at a 45° angle relative to the longitudinaldirection z. For example, provision is made of four reflector sections31, which reflect scattered light 3 from the light field 1 in fourlateral directions 5 to the collection reflector 32. The lateraldirections 5 are arranged in a preferably unevenly distributed manner atdifferent lateral angles with respect to the x-y plane, as shownschematically in FIG. 2A. From each reflector section 31, an image ofthe scattered light 3 generated in the light field 1 is reflected to thedetector camera 11 via the collection reflector 32. The reflectorsections 31 have advantages in terms of preventing background noise fromsecondary scattering, as the latter is conducted out of the arrangementof reflector sections 31.

The detector device 10 comprises a single detector camera 11 having adetector array 12, e.g., a Sony ICX285 CCD chip, and a camera objective13. The scattered light images 6 can be recorded with a uniform detectorarray 12. Pixel groups of the detector array 12 provide a plurality oflinear detector arrays or a plurality of planar detector arrays for theimage recording of the scattered light 3. As an alternative, single,separate detector arrays can be provided for the image recording of thescattered light 3. The detector device 10 can be equipped with acolor-sensitive detector array 12 and/or a spectrally selective filterdevice (not illustrated) for a spectrally selective image recording ofthe scattered light 3. With the camera objective 13, the detector camera11 displays the two-dimensional intensity distribution of the scatteredlight 3 in the area of the light field 1 via the planar mirrors of thereflector sections 31 and of the collection reflector 32. The boundarylines of the scattered light 3 give an impression of the pharoid beampath of the viewing field of the camera. A plurality of views of thescattered light of the light field 1 from different lateral directionsare thus displayed on the detector array 12 via the various mirrors. Asshown schematically in FIG. 2B, for example, four scattered light images6 are generated on the detector array 12 with the camera objective 13.The detector camera 11 sends an image signal representing the scatteredlight images 6 to the reconstruction device 20.

The reconstruction device 20 comprises a computer unit, which isdesigned to carry out a computer tomographic reconstruction process onthe basis of the image signals of the detector camera 11. Thereconstruction device 20 calculates the field density of the scatteredlight 3 in the light field 1 from the scattered light images 6 and theknown geometry of the deflector device 30, in particular thedistribution of the lateral angles 5 of the reflector sections 31.

The tomographic reconstruction yields a three-dimensional model (3D dataset) of the intensity distribution of the scattered light 3 in thedisplayed volume area of the light field 1. This three-dimensionalintensity distribution of the scattered light 3 is a function of thelocal radiation intensity of the light field to be measured. Theintensity distribution model characterizes the spatial brightnessdistribution of the light field, and it advantageously containsconsiderably more information about the light field 1 than any of theprojective two-dimensional camera views and more information than aseries of two-dimensional intensity projections of the measurement areaof the light field 1 than can be measured according to U.S. Pat. No.8,988,673 B2, and even more information than a series of intensityprofiles measured invasively and directly with an imaging detectorstanding perpendicular to the z axis.

FIG. 1 furthermore shows that the reconstruction device 20 can beequipped with an analyzer device 21 and a display device. With theanalyzer device 21, it is possible to calculate additional beamparameters (e.g., the intensity or the location of the focus) using thethree-dimensional field density of the scattered light 3 in the lightfield 1. The determined parameters can be used as error values forregulating the laser source 210 in order to set prespecified beamparameters, which are predefined by, for example, a schematically showncontrol device 50.

Shown schematically in FIG. 3 are features of another embodiment of theradiation field measuring device with a detector device 10, which has aplurality of detector cameras 11 (multi-camera arrangement). Thedetector cameras 11 are uniformly distributed in radial directionsaround the light field 1. Provision can be made for a non-uniformdistribution of the detector cameras 11 as an alternative. Each of thedetector cameras 11 is arranged for recording a scattered light image ofthe light field 1. In this case, it is possible to dispense with thedeflector device 30 shown in FIG. 1. Instead of several partial images,several individual camera images from different lateral directions formon the detector array 12 (FIG. 2B). The image signals of the detectorcameras 11 undergo tomographic reconstruction in a reconstruction device(not shown in FIG. 3).

FIG. 4 shows a variant of the embodiment of the radiation fieldmeasuring device 100 according to the invention having a single detectorcamera 11 (FIG. 1), in which the arrangement of reflector sections 31 ofthe deflector device 30 is replaced with an axicon reflector 33 (singlehollow cone-shaped reflector). The axicon reflector 33 is designed suchthat numerous images of the scattered light 3 of the light field 1 aredisplayed from various lateral directions on the detector array 12. Theuse of the axicon reflector 33 confers further advantages in connectionwith compressive sensing (CS), since the axicon measurements account forthe “incoherence” required in the CS sense in that they display theprojection of the scattered radiation of the radiation field, along thelongitudinal extension thereof, in the beam direction with continuouslyincreasing spatial resolution.

FIG. 4 notwithstanding, the axicon reflector 33 can be replaced by twoaxicon partial reflectors, which are obtained by dividing the axiconreflector 33 in half in the longitudinal direction of the light field 1and putting the halves together, wherein one partial reflector isrotated 180°. This variant of the invention can offer advantages bycompensating potential non-uniform spatial resolution on the axiconreflector 33.

Another variant of the radiation field measuring device 100 with asingle detector camera 11 is shown in FIG. 5, in which the axiconreflector is replaced by an arrangement of strip-type planar mirrors 34,which are arranged fan-like on a hollow cone surface. In this case,numerous images of the scattered light 3 of the light field 1 aregenerated on the detector array 12 from different lateral directions.

FIGS. 6 and 7 illustrate the application of the embodiment according toFIG. 5 for the characterization of a non-collimated light field 1. Thediameter of the light field 1 passes through a minimum at a focus 7.According to FIG. 6, the scattered light 3 from the measurement section4, which contains the focus 7, is diverted to the detector camera 11 bythe deflector device. The tomographic reconstruction of the fielddensity of the scattered light 3 in the light field 1 directly yields acharacterization of the focus 7. As an alternative, the scattered light3 can be captured in a measurement section 4 at a distance from thefocus 7, as shown in FIG. 7. The three-dimensional reconstruction of thefield density of the scattered light 3 permits a wave front analysis andascertainment of the propagation properties of the light field 1, inparticular the geometric form thereof, and thus indirectly also yields acharacterization of the focus 7 and the position thereof. The embodimentof FIG. 7 can be modified so that the focus 7 is located outside of thedeflector device 30. The focus 7 on, for example, the surface of amaterial to be processed can thus be advantageously examined in acontact-free and non-invasive manner.

Features of a modified embodiment of a radiation field measuring deviceaccording to the invention are shown in FIGS. 8A and 8B, in which thelongitudinal direction z of the light field 1 runs perpendicular to thedrawing plane. In both cases provision is made of a deflector device 30having a plurality of reflector sections 31 (planar mirrors), whichdivert scattered light 3 from the light field 1 to the detector camera11 of the detector device 10. For example, four reflector sections 31are provided.

The reflector sections 31 are arranged with surfaces parallel to thelongitudinal direction z such that a center line of each reflectorsection 31 forms a tangent to an ellipse in the x-y plane, wherein thedetector camera 11 (FIG. 8A) or an imaging objective 14 of a flexible orrigid image conducting fiber bundle 15 (FIG. 8B) is located in one focusof the ellipse and the light field 1 is located in the other focus ofthe ellipse. Five different lateral directions and accordingly fivedifferent scattered light images arise in conjunction with the directcamera perspective on the light field 1. According to FIG. 8A, thescattered light images are simultaneously recorded with the detectorcamera 11 directly via the camera objective 13. According to FIG. 8B,the scattered light images are recorded with the detector camera 11without an objective via the imaging objective 14, the image conductingfiber bundle (light wave conductor bundle with aligned fibers) 15 and arelay optic 16. The image signals of the detector camera 11, whichcontain the scattered light images, are sent to the reconstructiondevice (not illustrated).

The embodiment according to FIG. 8B has the advantage that the detectorcamera 11 can be arranged at a distance from the measurement section 4so that interfering conditions in the measurement section 4(electromagnetic interference fields or extreme temperatures, forexample) cannot affect the detector camera 11.

The planar reflector sections 31 of FIGS. 8A and 8B can be replaced withcurved reflector sections 31, as illustrated by way of an example inFIGS. 9 and 10. The aspherically curved reflector sections 31, whichpreferably comprise off-axis ellipsoids or paraboloids, have an imagingand light-collecting effect. The curved reflector sections 31 arearranged with their center lines on the ellipse described above withreference to FIG. 8. Scattered light 3 from the light field 1 isdisplayed on a single detector camera 11 having an entocentric objective13 (FIG. 9) or on two detector cameras 11 having entocentric objectives13 (FIG. 10).

The curved reflector sections 31 act like a field lens (or a fieldmirror), which is provided in object-side telecentric objectives betweenthe object and the camera. In this case, telecentric also means moreparticularly that there are not any distance-related imaging scalechanges. The internal shutter for inducing telecentricity that istypical of telecentric objectives is not illustrated in FIG. 9. Theadvantages of the arrangement of curved reflector sections 31 as a fieldmirror lie firstly in the high numerical aperture of the imaging opticsformed by the reflector sections 31 and the enhanced light intensity,and secondly in the telecentric effect in relation to the light field 1to be measured. This advantageously permits a lesser distance-dependentdistortion compared to simpler arrangements with planar mirrors ormulti-camera arrangements according to FIG. 3.

The object-side telecentric imaging of the scattered light 3 can giverise to a construction-related reduction in light intensity. Tocompensate for this, the arrangement of reflector sections 31 accordingto FIG. 9 can be modified so as to achieve a compromise betweensufficiently high light collection capacity and sufficient depth offield on one hand and sufficient telecentricity on the other hand.According to a further modification of the embodiment of FIG. 9, thereflector sections 31 could be replaced by an axicon reflector withelliptical curvature.

Embodiments of the invention in which the deflector device 30 comprisesdioptric elements, in particular lenses 35, 36 and/or prisms 37, areillustrated in FIGS. 11 and 12. FIG. 11 shows the deflector device 30with two lenses 35, 36, which are made of, e.g., quartz glass and whichform an f-theta arrangement jointly with the objective of the detectorcamera 11. In a manner similar to an axicon reflector, the lenses 35, 36provide a continuous image that contains scattered light 3 from allcaptured lateral angles side by side, wherein the individual scatteredlight images are extracted computationally from the continuous image forthe tomographic reconstruction. Advantageously, a larger angle range ofthe scattered light 3 from the light field 1 is captured by the f-thetaarrangement in FIG. 11 than by the multi-prism 37, which only displaysscattered light 3 in at most a half space of 180° or less (FIG. 12).Moreover, an object-side telecentric display is also achievable with thef-theta arrangement.

According to FIG. 12, the deflector device 30 is shown with amulti-prism 37 made of, for example, quartz glass, which is formed forimaging from five lateral angles (perspectives). The arrangementaccording to FIG. 12 can be advantageous if due to space constraints,for example, the radiation field measuring device is only to be arrangedon one side next to the light field 1 to be examined.

FIGS. 13 and 14 illustrate further embodiments of the invention, inwhich the deflector device 30 comprises dioptric elements in the form ofa simple prism 38 or multi-prism 39, which are made from, for example,quartz glass. Scattered light 3 from the light field 1 is diverted viathe prism 38 or multi-prism 39 to the detector camera 11, the imagesignal of which is sent to the reconstruction device (not illustrated).These embodiments have advantages because of the simple construction ofthe deflector device 30.

The damage threshold of an optical element is dependent on the materialof the optical element and in the case of quartz glass, is 1 MW/cm² forcontinuous laser light, for example, or 1 to 5 GW/cm² for pulsed laserlight (10 ns pulse length, laser intensity at 1064 nm: 20J/cm², pulserepetition frequency 100 Hz), for example. If the field density of theexamined light field 1 is below the damage threshold of opticalelements, in particular ones made of glass, the beam rotator 40 shown inFIG. 15 can be provided with a rotatable Dove prism 41 (or withrotatable mirrors, not illustrated) in order to capture scattered light3 corresponding to different lateral directions with the detector camera11. With the rotatably-mounted Dove prism 41, the light field 1generated by the laser source 210 can be rotated about the longitudinaldirection z, wherein an image recording takes place for each setrotation angle. Furthermore, a background screen 17 is provided, whichcomprises, e.g., a blackened metal or plastic panel and forms a darkbackground for the camera image behind the light field 1, whichbackground minimizes reflections of the scattered light incidentthereon.

The Dove prism 41 advantageously does not change the beam direction;instead it rotates itself about the beam axis z, about an angle. Thelaterally exiting light field 1 thus rotates about the double angle. TheDove prism 41 is turned 180° in order to permit the light field 1 torotate 360°. The detector camera 11 can thus record scattered lightimages with a freely selectable number of perspectives of the lightfield 1. With this measurement geometry, the background advantageouslyremains constant for all lateral directions.

FIG. 16 schematically illustrates the steps of the method for thecharacterization of a light field 1, which passes through a medium 2. Inthe medium 2, scattered light is generated by Rayleigh scattering of thelight field 1 on atoms or molecules of the medium 2. The intensity ofthe Rayleigh scattering I_(R) is given according to

I _(R) =I ₀(k/λ ⁴)(1+cos² Θ)

from the intensity of the light field Io, a constant k, the wavelength λand the angle Θ relative to the longitudinal direction of the lightfield. Accordingly, in addition to obtaining information on theintensity of the light field Io, the wavelength dependency of thescattered light can also be used to characterize the light field 1.Scattered light images of the generated scattered light are recorded atdifferent lateral angles relative to the longitudinal direction of thelight field. The scattered light images provide projections of thescattered light generated by the light field 1, which undergotomographic reconstruction. As a result, the 2D or 3D field density ofthe scattered light in the light field 1 is calculated, followed by ananalysis to determine properties of the light field such as the beamprofile or the shape of the wave front, for example.

The features of the invention disclosed in the present description, thedrawings and the claims, individually as well as in combination or insubcombination, can be essential to the realization of the invention inits different designs.

1-31. (canceled)
 32. A method for characterizing a radiation field (1)that passes through a medium (2) in a longitudinal direction (z), usinga radiation field measuring device (100), comprising the steps:recording an image, by means of a detector device (10), of scatteredradiation (3), which is generated in the medium (2) by the radiationfield (1) and is directed in a multiplicity of lateral directions thatdeviate from the longitudinal direction (z), and characterizing of theradiation field (1) with a reconstruction device (20) using imagesignals of the detector device (10), wherein reconstructing the imagewith the reconstruction device (20) carries out a tomographicreconstruction of a field density of the scattered radiation (3) in theradiation field (1).
 33. The method according to claim 32, in which thereconstruction device (20) carries out a non-analytical, in particularstatistical or algebraic, tomographic reconstruction of the fielddensity of the scattered radiation (3).
 34. The method according toclaim 33, in which the reconstruction device (20) carries out thetomographic reconstruction of the field density of the scatteredradiation (3) by means of an iterative algorithm.
 35. The methodaccording to claim 33, in which the reconstruction device (20) carriesout a statistical tomographic reconstruction of the field density of thescattered radiation (3), wherein the statistical tomographicreconstruction is based on a statistical model with an objectivefunctional to be minimized, the tomographic data mismatch term of whichaccounts for the noise characteristics of the measurement data.
 36. Themethod according to claim 35, in which the objective functional containsan Lp-norm term with (0≤p<2) and/or a Bayesian regularization term. 37.The method according to claim 32, in which the image recording ofscattered radiation (3) takes place such that the lateral angles aredistributed in such a way that the components of the recorded scatteredradiation (3) running perpendicular to the longitudinal direction (z)span a measurement range of 180° to 360°.
 38. The method according toclaim 37, in which the image recording of scattered radiation (3) takesplace such that the components of the recorded scattered radiation (3)running perpendicular to the longitudinal direction (z) are unevenlydistributed in the case of an even number of lateral directions and ameasurement range over 360°, and evenly distributed otherwise.
 39. Themethod according to claim 32, in which the image recording of scatteredradiation (3) takes place in at least 2 lateral directions, inparticular at least 3 lateral directions, and/or the image recordingtakes place in a spectrally selective manner.
 40. The method accordingto claim 32, in which the field density of the scattered radiation (3)is reconstructed in the forward and backward projection process of thetomographic reconstruction, with an illumination background of theradiation field (1) taken into account.
 41. The method according toclaim 32, comprising the steps tomographic reconstruction of a layersection of the field density of the scattered radiation (3), andconversion of the field density of the scattered radiation (3) into atwo-dimensional intensity distribution of the radiation field (1). 42.The method according to claim 32, comprising the step tomographicreconstruction of a field density of the scattered radiation (3) in athree-dimensional volume section, which comprises at least twojuxtaposed layer sections.
 43. The method according to claim 32,comprising the step deflection of the scattered radiation (3) along themultiplicity of lateral directions with the deflector device (30) ontothe at least one detector camera (11).
 44. The method according to claim32, comprising the steps rotation of the radiation field (1) about thelongitudinal direction (z) with the beam rotator, and image recording ofthe scattered radiation (3) with a single detector camera (11), whereinfor the image recording of scattered radiation (3) in the multiplicityof lateral directions, the radiation field (1) is rotated with the beamrotator into different rotational positions relative to the detectorcamera (11).
 45. The method according to claim 32, comprising the stepascertaining an intensity distribution of the radiation field (1) on thebasis of the reconstructed field density of the scattered radiation (3).46. The method according to claim 32, comprising the step of capturingat least one of the beam parameters, which comprise pulse energy orpulse energy density of the radiation field (1) in the case of a pulsedradiation field (1), field density of the radiation field (1) in thecase of a continuous radiation field (1), geometric properties of theradiation field (1), in particular beam diameter, divergence angleand/or beam shape, properties of the beam waist of the radiation field(1), in particular radius, position along the longitudinal direction(z), and/or shape of the focus in transaxial section, spatial locationof the radiation field (1) in the medium (2), coherence properties ofthe radiation field (1), wave fronts of the radiation field (1),Rayleigh lengths of the radiation field (1), and diffraction indexes, M²and beam propagation factors k of the radiation field (1).
 47. Themethod according to claim 45, in which provision is made for continuouscapturing of the at least one beam parameter and the temporal stabilitythereof.
 48. The method according to claim 45, comprising the stepcalculation of a beam propagation, in particular by means of wave frontanalysis.
 49. The method according to claim 48, comprising the stepcalculation of a focus position of the radiation field (1).
 50. Themethod according to claim 32, in which the field density of thescattered radiation (3) is reconstructed with particles in the mediumtaken into account, which particles would lead to artefacts of thereconstructed field density if they were not taken into account.
 51. Themethod according to claim 50, in which provision is made for a serialimage recording, which comprises a plurality of sequential imagerecordings, and artefact-bearing scatter events, which arise fromparticles in the medium, are eliminated by an analysis of the serialimage recording.
 52. The method according to claim 32, comprising thestep providing the medium (2) in the radiation field measuring device ina particle-free state.
 53. The method according to claim 32, comprisingthe step capturing a volumetric particle distribution in the radiationfield (1).
 54. The method according to claim 32, comprising the stepsmonitoring and/or controlling a radiation source with which theradiation field (1) is generated.
 55. The method according to claim 54,in which the radiation source is used for laser-supported materialprocessing in cutting and joining technologies or in manufacturing insemiconductor technology, or in therapy and/or surgery by means of laserradiation.
 56. The method according to claim 54, in which the radiationsource contains a setting device with which the beam parameters of theradiation field (1) can be varied, wherein the setting device iscontrolled according to an intensity profile of the radiation field (1)along the longitudinal direction (z), more particularly in the focus ofthe radiation field (1).
 57. The method according to claim 54, in whichthe radiation source contains a focusing device, and the focusing deviceis controlled according to the position of the focus along thelongitudinal direction (z).